Power control for induction heating device



Oct. 20, 1964 R. A. SOMMER 3,153,717

POWER CONTROL FOR moucnou HEATING DEVICE Filed April 9. 1962 FIG. I

INVEN TOR.

RICHARD A. SOMMER ATTORNEY United States Patent Office 3,153,717 PlllWER CQNTROL FOR KNDUCTEUN l-liEATlNG DEVICE Richard A. Summer, Farina, Qhlo, assignor to The (lhio Crankshaft Company, Cleveland, Ohio, a corporation of Ohio Filed Apr. 9, 1962, her. No. 186,017 4 tilaims. (Cl. 2l3 lth75) This invention pertains to the art of induction heating and more particularly to an induction heating device provided with a power density control means.

The invention has particular applicability to the art of heating billets such as those used in a forging operation, and it will be described with particular reference thereto; however, it is to be appreciated that the invention has much broader applications and may be utilized in a variety of induction heating apparatus.

it has become common practice to heat a forging billet or similar workpiece by inserting the end to be heated into an induction heating coil having an axial workpiece receiving passage. To assure proper heating of the innermost end of the workpiece, a magnetically permeable, laminated core is provided which extends from the rear of the induction coil up to the innermost end of the workpiece. In practice, the length of the workpiece often varies substantially from job to job. It has been found that as the workpiece becomes shorter and the portion of the induction coil occupied by the magnetically permeable, laminated core increases, it is more diflicult, and in some cases impossible, to induce a sufficient power density in the workpiece to cause proper heating of the workpiece in a time that is economical. The decrease of the power density in the shorter workpiece results from an increase in the total impedance of the induction coil because of a greater section of the induction coil which is magnetically coupled to the laminated core. By increasing the total impedance of the coil through coupling the induction coil to a longer section of the laminated core, the output voltage of a power source energizing the induction coil is not adequate to cause a sufficient current flow in the coil to produce the required power density in the workpiece.

To prevent this decrease in the power density as the length of the workpiece decreases, it has heretofore been necessary to shorten the elfective length of the coil by providing either separate induction heating coils for each workpiece length or by providing a tapped induction coil, either of which solution to the problem is expensive and inconvenient.

The present invention relates to an induction heating device that includes a coil and a means for controlling the power density of the coil that corrects the abovementioned dimculties and others and which results in a structurally sound and highly dependable heating device.

In accordance with the present invention, an induction heating device comprised of a coil having an axial workpiece receiving passage and a magnetically permeable core adjustably positioned in the passage to accommodate workpieces of variable lengths is provided with a power density control means comprised of a high electrically conductive sleeve generally concentric with the coil and adjustably movable axially along the core to lower the impedance of the coil adjacent the sleeve and increase the power density in the remainder of the coil.

The primary object of the present invention is to provide an induction heating device which will uniformly heat workpieces of various lengths and which is economical to produce and dependable in operation.

Another object of the present invention is to provide an induction heating device for heating workpieces of various lengths having a coil and a means for controlling the power density in the workpiece.

Still a further object of the present invention is to provide an induction heating device having an induction coil which will uniformly heat workpieces of variable lengths without providing a number of taps along the length of the coil.

Another object of the present invention is to provide an induction heating device having an induction heating coil that will heat workpieces of variable lengths with approximately the same power density in the workpiece without necessitating complicated external mechanisms.

Still a further object of the present invention is to provide an induction heating device for pieces of variable lengths which device comprises an induction coil sub-- stantially the same as presently used and a simplified means for controlling the power density in the workpiece.

Another object of the present invention is to provide an induction heating device for workpieces of various len ths which comprises a means for controlling the power density of the induction heating coil.

Another object of this invention is to provide an induction heating device comprised of an induction heating coil, a magnetically permeable core, and an electrically conductive sleeve slid-able with respect to the core and the coil to adjust the power density of that portion of the coil not occupied by the magnetically permeable core.

These and other objects and advantages will become apparent from the following description used to illusrate the preferred embodiment of the invention as read in connection with the accompanying drawing in which:

FIG. 1 is a cross-sectional view illustrating somewhat schematically an induction heating device used to heat a relatively long workpiece; and

FIG. 2 is a cross-sectional View illustrating somewhat schematically the preferred embodiment of the present invention used to heat a relatively short workpiece.

Referrin; now to the drawing wherein the showings are for the purpose of illustrating a preferred embodiment of the invention and not for limiting same, FIG. 1 shows an induction heating device A similar to the heating devices used to heat billets prior to performing a forging operation. The'heating device is comprised of an induction heating coil ltl which in practice is encased in an insulating material 12 having a concentric axially extending passage l4 adapted to receive a workpiece 16, a magnetically permeable core 13 which extends into the passage l to a position adjacent the innermost end of the workpiece to assure proper heating of this end, a generator 2th or another appropriate source of alternating current and a conventional power factor correcting capacitor 22. The induction heating coil 10 is disclosed as a helically wound coil of a cylindrical conductor; however, in practice, the cross section of this conductor may be varied and internal or external cooling means may be provided to dissipate heat generated during the operation of the coil. The magnetically permeable core Elli is preferably corrposed of a number of iron laminae and has a cross-sectional area generally concentric with the axially extending passage 14. The laminated construction of the core 13 substantially prevents flow of currents in the core.

In operation of the induction heating device A disclosed in FIG. 1, the induction coil 10 is essentially divided into two sections, the first represented by L is the section of the coil magnetically coupled to workpiece 16 and the second represented by ]L is the section of the coil magnetically coupled to the core 13. The material of the workpiece and the core determines the impedance along the coil in the separate coil sections. The impedance per length of coil in section L being substantially less than the impedance per length of coil in section L The relationship of these lengths determines the power density in the workpiece. Tie power density determines the temperature and the time of heating of the workpiece. By using the induction heating device disclosed in H6. 1, a uniform power density can be obtained if the length of the workpiece does not vary and the construction of this device is relatively inexpensive.

If the workpiece is shortened, the length of coil section L decreases and the length of coil section L increases correspondingly. This increases the section of the coil which is characterized by relatively high impedance, i.e., section L and the power density along the length of the workpiece, i.e., opposite section L is correspondingly decreased to prevent proper heating of the workpiece. The present invention is directed to an induction heating device which allows control of the power density in the workpiece to maintain a constant heating characteristic of the workpiece. One structural embodiment of this device is disclosed in FIG. 2 wherein a high electrically conductive sleeve 30 is positioned around a portion of the magnetic core 18. The sleeve 34 may be produced from various low resistance materials, e.g., silver, aluminum or copper; however, in practice a copper sleeve has proved economical and quite satisfactory. The sleeve is usually water cooled by an appropriate means although no cooling means is shown. Since the sleeve 31; is highly conductive, the sleeve in essence short circuits the section of the induction coil opposite the sleeve and lowers the impedance of the coil thereat. Stated in another way, t

highly conductive sleeve 3d prevents the induction coil.

from magnetically coupling with the core 18 so the core does not produce a high impedance Within the coil adjacent the sleeve 3t). The section of the coil opposite the sleeve is represented by L and the impedance along this coil section is substantially less than the impedance in sections L and L This results in a coil that has substantially the complete voltage drop across section L and L The sleeve is so received over the core and within the passage 14- that it may slide with respect to both of these elements.

In operation of the induction heating device A disclosed in FIG. 2, which is a preferred embodiment of the present invention, the ratio between the length of section L and the length of section L determines the power density in the length of the workpiece 16. Assuming that the sleeve 30 was not positioned around the core, the power density directed to the workpiece would be substantially decreased since the length of coil in section L has increased while the length of the coil in section L has decreased. By sliding the sleeve 39 toward the workpiece with any appropriate means, the length of section L is decreased. Thus, by moving the sleeve toward the workpiece, ratio of the length of sections L to the length of section L may be controlled to obtain a power density in the workpiece substantially identical to the power density of a longer workpiece as shown in FIG.1.

Although the present invention has been described in an embodiment wherein the power density in the workpiece is to remain constant for various workpiece lengths, it is within the contemplation of this invention to use the sleeve 30 as a means for varying the power density in the workpiece. This device then may be used to provide a faster or slower heating rate and a substantially dijterent temperature it it is desired to change either of these perimeters.

The present invention has been described in connection with a preferred embodiment; however, it is not intended that the scope of the invention should be limited by the structural details found in the preferred embodiment. The invention may be used in more and broader applications without departing from the spirit and scope or" the appended claims.

Having thus described my invention, I claim:

1. An induction heating device comprised of an induction coil having an axial passage, a high magnetically permeable core slidably mounted in a first portion of said passage, and a workpiece in the remaining second portion of said passage, the power density in said workpiece being determined by the relationship of the length of said coil magnetically coupled to said workpiece to the length of said coil magnetically coupled to said core, the improvement comprisin a means for controlling the power density in said workpiece, said means comprising, a highly electrically conductive sleeve slidable in said first portion of said passage along said core and spaced from said coil to change the length of said coil magnetically coupled to said core.

2. The improvement as defined in claim 1 wherein said sleeve is comprised of copper.

3. An induction heating device for heating a workpiece comprised of an induction coil having an axial passage, a high magnetically permeable core magnetically coupled to said coil and slidably mounted in a first portion of said passage, said core defining a workpiece receiving second portion of said axial passage, and a workpiece magnetically coupled to said coil and positioned in said second portion, the power density in said workpiece being determined by the relationship of the length of said cell magnetically coupled to said workpiece to the length of said coil magnetically coupled to said core, the improvement comprising: a means for controlling the power density in said workpiece, said means comprising, a hollow electrically conductive member having low resistance substantially concentric with said core and slidably mounted with respect thereto, said member adapted to vary the length of said coil magnetically coupled to said core.

4. An induction heating coil having a workpiece receiving axial passage and a means for controlling the power density in said workpiece, said means comprising, said coil divided into a first, second and third section, said sections extending along the length or" said coil, said workpiece in said passage and adjacent said first coil section, a high magnetically permeable core in said passage and adjacent said second and said third coil sections, an electrically conductive sleeve slidably received on said core, said sleeve opposite said third coil section, and means for moving said sleeve along said core changes the power density in said. workpiece by changing the length of the second and third coil sections.

References Cited in the file of this patent UNITED STATES PATENTS 2,448,008 Baker Aug. 31, 1948 2,820,128 McArthur Ian. 14, 1958 2,856,499 Stanton et al. Oct. 14, 1958 FOREIGN PATENTS 1,068,832 Germany Nov. 12, 1959 

1. AN INDUCTION HEATING DEVICE COMPRISED OF AN INDUCTION COIL HAVING AN AXIAL PASSAGE, A HIGH MAGNETICALLY PERMEABLE CORE SLIDABLY MOUNTED IN A FIRST PORTION OF SAID PASSAGE, AND A WORKPIECE IN THE REMAINING SECOND PORTION OF SAID PASSAGE, THE POWER DENSITY IN SAID WORKPIECE BEING DETERMINED BY THE RELATIONSHIP OF THE LENGTH OF SAID COIL MAGNETICALLY COUPLED TO SAID WORKPIECE TO THE LENGTH OF SAID COIL MAGNETICALLY COUPLED TO SAID CORE, 